1. Field of the Invention
The present invention relates to a shape measuring apparatus.
2. Description of the Related Art
An optical external dimension measuring apparatus is available as a shape measuring apparatus. The optical external dimension measuring apparatus measures the shape of an object on the basis of an image (that is, a project image) of the object that is projected onto an image sensor by directing a parallel beam towards the image sensor beyond the object. The optical external dimension measuring apparatus comprises optics including a projector which includes an optical source, a bar lens, and a lens, and an optical receiver which includes a cylindrical lens and an image sensor.
An object to be measured that is placed between the projector and the optical receiver has, like a rod, a short length in the direction of an optical axis in which a parallel beam extends (that is, a short length in the optical axis direction of the object to be measured).
However, it is difficult to irradiate the object with a light beam that is exactly parallel. When the shape of an object having a long length in the direction of the optical axis is measured, the light from a lens is scattered at many points at a surface of the object along the direction of the optical axis. Therefore, an edge of an image of the object projected onto the image sensor is no longer distinct, thereby increasing the extent of blurring of the image. Consequently, it becomes difficult to identify the true edge, thereby reducing the precision with which the shape of the object is measured.
In the related shape measuring apparatus, a monochromatic (single-wavelength) light-emitting diode (LED) having a relatively excellent coherence is used as the optical source (light source). However, when an object to be measured has a long length in the direction of the optical axis, a projection optical path is long. Therefore, when the coherence is excellent, the number of points where a parallel beam is diffracted is increased, as a result of which a number of diffraction fringes (that is, interference fringes) are produced one above another near and along the true edge of the image of the object that is projected on the image sensor. Consequently, an image processing operation for preventing erroneous detection of the true edge becomes complicated, and a load on an image processing system is increased, thereby reducing the precision with which the shape is measured and increasing the time required for the measurement.
In the above-described related shape measuring apparatus, a flash lamp is also used as the light source. The flash lamp (xenon flash lamp) is a white light source exhibiting no coherence unlike an LED. In terms of size, however, it is difficult to obtain a point light source required to form a light beam that is close to an exactly parallel beam to the greatest degree possible. As the size of the light source increases, the extent of blurring of the edge of the image of the object is increased. In addition, the flash lamp has poor durability.